1. Technical Field
The present invention relates to power transmission control devices, power transmission devices, power reception control devices, power reception devices, electronic apparatuses, and contactless power transmission systems.
2. Related Art
In recent years, contactless power transmission (non-contact power transmission) which utilizes electromagnetic induction to enable power transmission without metal-to-metal contact has been attracting attention. Among application examples of contactless power transmission, an application thereof to charging a portable telephone, a household appliance (for example, telephone handset), and the like has been proposed.
When a contactless power transmission system is to be put in practical use, a reduction of power consumption of the contactless power transmission system is an important issue. For example, the power level of transmission power may be kept low during periods other than a period in which a load current is being supplied to a load connected to the secondary side, which contributes to suppression of wasteful power consumption. As a method of reducing the power level of transmission power, reducing the power supply voltage (operation voltage) of the driver circuit (power transmission driver) that drives the primary coil is effective.
For example, JP-A-2005-86866 describes a technology in which a commercial AC power supply is converted to a DC voltage by an AC/DC converter circuit, the level of the DC voltage is converted to a desired level by a DC/DC converter circuit, the obtained DC voltage is converted to an AC signal by a DC/AC converter circuit, and then the AC signal is supplied to a coil transformer (primary coil). By using this technology in a power transmission device of a contactless power transmission system, the power supply voltage (operation voltage) for the primary coil can be reduced.
In the power transmission device of the contactless power transmission system described above, a power transmission driver having a plurality of switching elements (for example, power MOS transistors) may be used. The power transmission driver AC-drives the primary coil (in this case, the transmission driver functions as a DC/AC converter circuit) based on a power supply voltage (for example, a DC voltage obtained by AC-to-DC converting the commercial power supply).
Specifically, the transmission driver may be made of a full-bridge inverter (that includes four switching elements connected in a bridge configuration). On-loss is generated when each of the switching elements is turned on. For example, when the switching elements are composed of MOS transistors, on-loss of the MOS transistors is generated. Further, as parasitic diodes (body diodes) are present between sources and drains of the MOS transistors, on-loss would also be generated in the parasitic diodes (body diodes) through which regeneration current flows.
On-loss on each of the switching elements composing the power transmission driver (i.e., power loss of the switching element itself and power loss at the parasitic diode through which regeneration current follow) may be negligible when the power supply voltage (operation voltage) of the power transmission driver is high. However, as the power supply voltage of the power transmission driver lowers, the proportion of the on-loss of the switching elements with respect to the entire electrical power increases, and the on-loss becomes more prominent, which leads to a problem in that the efficiency of the power transmission driver (i.e., the conversion efficiency of the DC/AC converter circuit) is lowered.
In other words, in order to reduce the power consumption of a contactless power transmission system, the power supply voltage (operation voltage) of its power transmission driver may be lowered. However, with a reduction in the power supply voltage (operation voltage), on-loss relating to each of the switching elements forming the power transmission driver becomes more prominent, and therefore it is in effect difficult to lower the power supply voltage (operation voltage) of the power transmission driver to a desired voltage level, which poses a limitation to the reduction (control) of transmission power.